This invention relates to superconductors of the type including a superconductive intermetallic compound consisting of at least two elements in general, and more particularly to an improved method for manufacturing such a superconductor.
Intermetallic superconducting compounds consisting of two elements of the type A.sub.3 B, for example, Nb.sub.3 Sn or V.sub.3 Ga, and having an A-15 crystal structure are known to have very good superconductive properties and are distinguished in particular by a high critical magnetic field, a high transition temperature and a high critical current density. As a result, compounds of this nature are particularly well suited for superconducting coils used for generating strong magnetic fields such as those used for example, for research purposes. In addition, such coils find application in the superconducting magnets used in the suspended guidance of magnetic suspension railroads or in the windings of electric machines. More recently, ternary compounds such as, niobium-aluminum-germanium (Nb.sub.3 Al.sub.o.8 Ge.sub.0.2) have become of particular interest. However, since these compounds are very brittle, considerable difficulty is encountered in their manufacture into a form suitable for use in magnet coils or the like.
Several methods have been developed, making possible the manufacture of superconductors with, in particular, two-component intermetallic compounds in the form of long wires or ribbons at low diffusion temperatures. These methods which are particularly applicable to the manufacture of multi-core conductors using wires of Nb.sub.3 Sn and V.sub.3 Ga, embedded in a normal conducting matrix, are carried out by surrounding a ductile element of the compound to be produced in wire form, such as a niobium or vanadium wire in a sheath of an alloy containing a ductile carrier metal and other elements of the compound, e.g., a copper-tin alloy or a copper-gallium alloy. In particular, multiplicity of such wires can be embedded in a matrix of the alloy. The structure so obtained is then subjected to a cross section reducing process. This results, on one hand, in a long wire such as that required by coils, and on the other hand, the reduction of the diameter of the wire which is, for example, niobium or vanadium to a low size in the order of about 30 to 50 .mu.m. This is desirable in view of the superconductive properties of the conductor. In addition, the cross section reducing process obtains the best possible metallurgical bond between the wire and the surrounding jacket material of the alloy, without the occurance of reactions that lead to an embrittlement of the conductor. After the cross section-reducing process, the conductor, which consists of one or more wires and the surrounding matrix material, is subjected to heat treatment in such a manner that the desired compound is formed through the reaction of the wire material such as niobium or vanadium with the additional element of the compound contained in the surrounding matrix. This additional material would be for example, tin or gallium. During this process, the element contained in the matrix diffuses into the wire material and reacts with the latter forming a layer consisting of the desired compound. Processes of this nature are disclosed in German Offenlegungsschrift No. 2,044,660 German Offenlegungsschrift No. 2,052,323, and German Offenlegungsschrift 2,105,828.
In these prior art methods, wires consisting of one element of the compound to be produced such as niobium or vanadium, are embedded in the matrix material of an alloy containing a ductile carrier metal and the other elements of the compound, and are then deformed together with the matrix material. The matrix material, however, hardens very quickly during cross section-reducing cold-working and can then be deformed further only with great difficulty. Thus, in carrying out these prior art methods, it is necessary to anneal the conductor structure consisting of the wires and the matrix material relatively frequently during the cross section-reducing processing in order to heal the lattice dislocations produced in the matrix material. Although such annealing treatments can be carried out at temperatures and annealing times, which will not as a rule cause the superconductive compound to be produced, they are very time-consuming, particularly because of their frequent repetition. Because of this, various processes have been proposed in which the repeated annealing is avoided. In these processes, one or more cores of one ductile element of the compound to be produced, e.g., niobium or vanadium are embedded in a ductile matrix material such as copper, silver or nickel which does not contain an element of the compound to be produced. The resulting structure is then processed and does not require any intermediate annealing during the cross section reducing process. Thus, it can be drawn down very quickly into a fine wire containing very fine cores of vanadium or niobium using cold-drawing processes. After the last cross section reducing process step, the remaining elements of the compound to be produced, i.e., tin in the case of Nb.sub.3 Sn, or gallium in the case of V.sub.3 Ga, are then applied to the matrix material. This is done, for example, when using tin by immersing the wire briefly into a tin melt so that a thin layer of tin is formed on the matrix material. Alternatively, it is done by evaporating a tin layer onto the matrix material. Thereafter a heat treatment is performed in which the elements of the compound to be produced which are applied to the outside of the matrix material diffuse into and through the latter and form the desired superconductive compound through reaction with the cores. Heat treatment can be carried out in one or more steps. For example, in a first step, the tin applied to the copper matrix diffuses into the matrix and during a second step the compound Nb.sub.3 Sn is formed by reaction of the diffused-in tin with the niobium core. Processes of this nature are disclosed in "Applied Physics Letters" vol. 20, pages 443 to 445, (1972) and German Offenlegungsschrift No. 2,205,308.
Although these methods avoid the repeated annealing, they are not without difficulties and disadvantages. one disadvantage is that only relatively small amounts of tin, for example, can be applied to a matrix which consists of a material such as copper. If larger amounts of tin are applied to the copper matrix, then undesirable brittle intermediate phases of copper and tin are formed at the temperature required for the diffusion of tin or the conductor surface is attacked by the tin. As a result of this limitation, only a limited amount of tin is available for the formation of the intermetallic compound Nb.sub.3 Sn. At the temperatures required for the tin to diffuse into the copper matrix, the tin applied to the matrix surface also is at a temperature exceeding its melting point and can easily drip or run off from the matrix surface or at the very least distribute itself unevenly over the surface of the matrix. This can lead to large irregularities in the diffusion and subsequent formation of the Nb.sub.3 Sn layers. In addition, during heat treatment, care must be taken because of the tin which initially melts, to support the conductor in as opened a manner as possible, at least during the beginning of the heat treatment, in order that the melting tin layer does not touch other materials. Similar problems occur in the manufacture of conductors having other superconductor intermetallic compounds such as V.sub.3 Ga.
Thus, it can be seen that there is a need for an improved method of manufacturing conductors having superconductive intermetallic compounds which does not require repeated annealing during the cold drawing of the conductor for cross section-reducing but at the same time avoids the problems associated with applying a second element such as tin over the matrix after drawing. In particular, such a method should avoid having a melted element or compound on the outside during a heat treating process.